1. Field of the Invention
The present invention relates generally to optical filters, and more particularly to an optical filter assembly having selectable bandwidth, and to multispectral imaging systems using such filters.
2. Description of the Related Art
Birefringent filters such as the Lyot, Evans, and Solc designs are used as imaging spectral filters. These filters typically comprise several filter stages placed in optical sequence to produce an overall filter response having a desired free spectral range (FSR) and full-width at half maximum (FWHM). Tunable birefringent filters have been built using liquid crystal elements so that the center wavelength of the pass-band can be dynamically selected from within a tuning range that can be many times the filter's FWHM. Often, the tuning range is substantially the same as the filter's FSR. Tunable filters are used for multi-spectral imaging in applications such as the life sciences, remote sensing, and materials analysis, for example.
Birefringent filters that are selectable between an all-pass (white) state and an optically filtering state have been described by Miller in U.S. Pat. No. 5,892,612, and by Sharp in U.S. Pat. No. 5,990,996.
Multispectral imaging systems based on tunable birefringent filters are sold commercially by Cambridge Research & Instrumentation, Inc (Woburn, Mass.). These have a bandwidth which is determined by the birefringent filter design, and cannot be dynamically changed. For example, the Nuance VIS-20 has a bandwidth of 20 nm when it is tuned to 550 nm, and this bandwidth is approximately constant (in wavenumbers) as the system is tuned to different wavelengths over its 420-700 nm operating range. This system uses a filter with the Lyot design, but the pass wavelength is tunable because the retarder elements incorporate liquid crystal variable waveplates as tuning elements. Another model, the Nuance VIS-FL, uses a filter whose narrowest stage incorporates 3 tunable birefringent elements, to produce a passband which is a 5th order approximation to a flat-topped square passband. Its bandwidth is fixed by design, and is approximately constant (in wavenumbers) as the system is tuned to different wavelengths over its 420-700 nm range. Other models, such as the Nuance NIR-20 or the Nuance GNIR, are similar but operate in the infrared range at wavelengths up to 1050 nm.